This invention relates to methods for determining the thickness of thin films on small areas of structures used in microelectronics fabrication, e.g., near a semiconductor wafer""s edge or on a damascene-type structure.
During the fabrication of microelectronic devices, thin films of metals and metal alloys are deposited on silicon wafers and for use as electrical conductors, adhesion-promoting layers, and diffusion barriers. Microprocessors, for example, use metal films of copper, tungsten, and aluminum as electrical conductors and interconnects, titanium and tantalum as adhesion-promoting layers, and titanium:nitride and tantalum:nitride as diffusion barriers. Thickness variations in these films can modify their electrical and mechanical properties, thereby affecting the performance of the microprocessor. The target thickness values of these films vary depending on their function: conductors and interconnects are typically 3000-10000 angstroms thick, while adhesion-promoting and diffusion-barrier layers are typically between 100-500 angstroms thick. The deposition of each of these films must be controlled such that the film""s thickness is within a few percent (e.g., 5-100 angstroms, a value roughly equivalent to one or two seconds human fingernail growth) of its target value. Furthermore, the uniformity of the film over the surface of the wafer must be closely controlled in order to assure uniform behavior of the individual microprocessors and, consequently, high manufacturing yields. Because of these rigid tolerances, film thickness is often measured as a quality-control parameter during and/or after the microprocessor""s fabrication.
The metal films are often deposited and patterned in complex geometries and this complicates the measurement process. In a typical fabrication process, a titanium:nitride film is deposited over the entire surface of a silicon wafer. A tungsten film is then deposited onto the titanium:nitride film to leave an xe2x80x9cedge-exclusion zonexe2x80x9d, i.e., a small (about 1 or 2 mm) region where the titanium:nitride is exposed, near the wafer""s edge. The edge-exclusion zone prevents delamination of the tungsten film neat its edges. Near this region, the thickness of the tungsten film rapidly increases to its target value; this takes place over a distance of a few hundred microns. Without this rapid increase in film thickness, devices patterned near the wafer""s edge-exclusion zone will contain non-ideal tungsten films not having adequate thickness, and they will not meet specifications.
An example of a complicated film geometry recently introduced in commercial microelectronics fabrication is a xe2x80x9cdamascenexe2x80x9d or xe2x80x9cdual damascenexe2x80x9d structure. These structures, used especially to form copper conductors and interconnects, are typically formed by a multi-step process: copper is first deposited onto a wafer having a dielectric layer that has been etched to have a series of trenches; the wafer is then polished by chemical-mechanical polishing (CMP) to remove excess copper, leaving only copper-filled trenches. The resulting structure is typically a series of separated copper lines having a thickness of a few thousand angstroms, a width of about 0.5 microns, a periodicity of about 2 microns, and a length of several millimeters.
Measuring film thickness in and near the edge-exclusion zone and in damascene-type structures is difficult and impractical using conventional techniques. For example, blanket metal films are typically measured using a 4-point probe. Here, two separated pair of conducting probes contact the film; electrical resistance, as measured by the probes, relates to the film""s thickness. Because the spatial resolution of the 4-point probe is typically a few hundred millimeters, this instrument is impractical for both edge-profile and damascene-type structures. Moreover, a film""s resistance often depends on both its thickness and geometry, a complication that further reduces the accuracy of the 4-point probe when used to measure complex geometries. Another film-measuring instrument, called a stylus profilometer, drags a stylus needle over a sample, recording variations in topography. This instrument, however, is slow, cumbersome, sensitive to slight amounts of sample curvature, and inaccurate when used to measure relatively long distances (e.g., the hundreds of microns required for tungsten build-up near the exclusion zone).
In addition to these disadvantages, both 4-point probes and stylus profilometers require contacting and thus contaminating a sample. These instruments are therefore typically used on monitor or test samples, rather than samples containing actual product. Other methods for measuring the thickness of metal films, such as X-ray fluorescence and Rutherford backscattering, are non-contact, but are slow and have poor spatial resolution.
In general, in one aspect, the invention provides a method for measuring a structure that contains overlying and underlying films in a region where the overlying film""s thickness rapidly decreases until the underlying film is exposed (e.g., an edge-exclusion structure). The method includes the steps of: (1) exciting acoustic modes in a first portion of the region with at least one excitation laser beam; (2) detecting the acoustic modes with a probe laser beam that is either reflected or diffracted to generate a signal beam; (3) analyzing the signal beam to determine a property of the structure (e.g., the thickness of the overlying layer) in the first portion of the region; (4) translating the structure or the excitation and probe laser beams; and (5) repeating the exciting, detecting, and analyzing steps to determine a property of the structure in a second portion of the region.
In one embodiment, the exciting, detecting, analyzing, and translating steps are repeated to determine a property of the structure in multiple portions of the region. In one case, the above-mentioned steps are repeated in an edge-exclusion structure until the thickness of the overlying film is measured from where the underlying film is exposed to where the overlying film""s thickness is at least 80% of its average value. This particular method can be extended so that the steps are repeated in the structure until a diameter of the overlying film is measured. Typically, this xe2x80x9cdiameter scanxe2x80x9d embodiment includes repeating the above-mentioned step on each side of the overlying film""s diameter, and measuring multiple points near the center of the film.
In another embodiment, the exciting, detecting, analyzing, and translating steps are repeated until a property of the underlying film (e.g., the width of the edge-exclusion zone) is measured from where it is exposed to the edge of the wafer.
In typical embodiments: the overlying film is selected from a metal such as tungsten, copper, aluminum, and alloys thereof; the underlying film is selected from materials such as oxides, polymers, and metals such as titanium, titanium:nitride, tantalum, tantalum:nitride, and alloys thereof. These films are usually deposited on a silicon wafer.
The structure is typically measured using an optical method where the acoustic modes are excited with at least one optical pulse having a duration less than 1 nanosecond. In a particular embodiment, the exciting step features exciting time-dependent acoustic modes in the structure by directing a spatially periodic excitation radiation field defined by a wavevector onto the sample. The radiation field, for example, is formed by overlapping two optical pulses in time and space in or on top of the sample. The detecting step then includes diffracting probe radiation off a modulated optical or mechanical property induced on the sample""s surface by the acoustic modes. To determine thickness of the overlying layer, the density and acoustic properties of the overlying and underlying layers, the wavevector, and a frequency of the acoustic mode are analyzed (e.g., by comparing them to a mathematical model).
In another aspect, the invention features a method for measuring a structure comprising multiple thin, metallic, rectangular-shaped or linear regions, each having a width of less than 5 microns and being disposed between neighboring regions that include a second, non-metallic material (e.g., a damascene-type structure). The method includes the steps of: (1) exciting acoustic modes in at least one metallic, rectangular-shaped region by irradiating the region with a spatially periodic excitation field defined by a wavevector; (2) detecting the acoustic modes by diffracting a probe laser beam off a ripple morphology induced in the regions by the acoustic modes; and (3) analyzing the diffracted signal beam to determine a property of the structure (e.g., the thickness of the metallic, rectangular-shaped regions).
In a particular embodiment, the exciting step includes irradiating multiple metallic, rectangular-shaped regions with the excitation field. A probe laser beam is then diffracted off the surface ripple induced in each region by the acoustic modes. Thickness can be determined by analyzing a density and acoustic properties of the metal included in the region, the wavevector, and a frequency of the acoustic mode. Here, the width of the metallic, rectangular-shaped region or a distance separating consecutive regions may be used in the analysis. In still other embodiments, the signal beam can be further analyzed (e.g., by monitoring diffraction of the probe beam) to determine a width of the metallic, rectangular-shaped region or a distance separating consecutive metallic, rectangular-shaped regions.
In embodiments, each of the metallic, rectangular-shaped regions comprises copper, tungsten, aluminum, or alloys thereof, and have a width of less than 1 micron. The rectangular-shaped regions can also include more than one layer. For example, the trench may be lined with tantalum and then filled with copper.
In another aspect, the invention provides a method for measuring a structure comprising multiple thin, metallic, rectangular-shaped regions, each having a width of less than 1 micron and being disposed between neighboring regions comprising a second, non-metallic material. The method includes the steps of: (1) exciting acoustic modes in multiple metallic, rectangular-shaped regions by simultaneously irradiating the regions with a spatially periodic excitation field defined by a wavevector; (2) detecting the acoustic modes by diffracting a probe laser beam off a modulated optical or physical property induced in each of the regions by the acoustic modes; and (3) analyzing the signal beam to determine an average thickness of the metallic, rectangular-shaped regions irradiated by the excitation field.
The invention has many advantages. In particular, the method makes accurate measurements of film thickness in and near the edge-exclusion zone, in damascene-type structures, and in other small-scale structures. Measurements feature all the advantages of optical metrology, e.g., noncontact, rapid and remote measurement over a small region. The method collects data from a single measurement point having an area of between 10 and 100 microns in less than a few seconds. From these data film thickness in the small-scale structures is determined with an accuracy and repeatability of a few angstroms. For damascene-type structures, the method simultaneously measures the thickness multiple metal lines lying within the optical spot size with virtually no decrease in data quality, accuracy, or repeatability. For typical films used in a microelectronic device, the measurement determines thickness to within a fraction of a percent of the film""s true value.
In addition to thickness, the measurement determines the width of an exclusion zone, the diameter of the useable area on the wafer, the film""s slope near the edge-exclusion zone, and properties of damascene-type structures, such as the width and periodicity of the metal lines and the number of defects in the structure.
The optical system used to make these measurements is compact, occupying a footprint of about 2 square feet, and composed primarily of inexpensive, commercially available components.
Because of its small size, the optical instrument can be a stand-alone unit, or can be attached directly to a film-formation tool (e.g., a chemical-vapor deposition tool, a plasma-vapor deposition tool, a cluster tool, or a vacuum chamber) or a film-processing tool (e.g., a chemical-mechanical polisher). In these embodiments, the film-formation tool includes an optical port (e.g., a glass window) that is transparent to the excitation and probe radiation. Thus, during operation, the film-measuring instrument is oriented so that the excitation and probe radiation, and the diffraction signal, pass through the optical port.
Other features, aspects, and advantages of the invention follow from the following detailed description, and from the claims.